The specific aims of this study is to evaluate the safety and efficacy of lansoprazole in pediatric patients ages 1 to 11 years with gastroesophageal reflux disease (GERD); to compare the pharmacokinetic profile to that obtained in adults and to provide dosing recommendations for pediatric labeling. Lansoprazole (Prevacid) is a compound that inhibits gastric acid secretion. Its mechanism of action is to selectively inhibit the parietal cell membrane enzyme (H+,K+) -ATPase, or the "proton pump". Lansoprazole is approved for the short-term treatment (4 to 8 weeks) of several gastrointestinal diseases in adults, including symptomatic GERD. While lansoprazole can be beneficial for adults with these conditions, few studies have been conducted in children. This is an 8 to 12 week open label trial. Approximately 60 children are expected to be involved at ten centers across the U.S. The screening (pretreatment) period will last 7 to 10 days. During this time, parents (caregivers) will be given a diary to record the severity of their child's GERD symptoms and the frequency of antacid administration. A 24 hour pH evaluation and endoscopy with biopsies will be performed during the screening period. If, based on the results of the screening procedure, subjects are qualified they will enter the open-label treatment period.